Jeroen Demeyer on Wed, 03 Jun 2015 16:05:28 +0200

 Struggling with ellisogeny() and variable priorities

• To: pari-users <pari-users@pari.math.u-bordeaux1.fr>
• Subject: Struggling with ellisogeny() and variable priorities
• From: Jeroen Demeyer <jdemeyer@cage.ugent.be>
• Date: Wed, 03 Jun 2015 16:05:20 +0200
• Delivery-date: Wed, 03 Jun 2015 16:05:28 +0200
• User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

```This works:

```
gp> E = ellinit([Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(1, y^2 + 1), Mod(0, y^2 + 1)])
```gp> isog = factor(elldivpol(E,37))[1,1];
gp> yy = varhigher("yy")
%2 = yy
gp> xx = varhigher("xx")
%3 = xx
gp> ellisogeny(E, isog, 0, xx, yy);

But this (which should be equivalent, right?) does not:

```
gp> E = ellinit([Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(1, y^2 + 1), Mod(0, y^2 + 1)])
```gp> isog = factor(elldivpol(E,37))[1,1];
gp> yy
%2 = yy
gp> xx
%3 = xx
gp> ellisogeny(E, isog, 0, xx, yy)
***   at top-level: ellisogeny(E,isog,0,
***                 ^--------------------
*** ellisogeny: incorrect priority in ellisogeny: variable xx <= yy

```